Optical spectrometers are systems that enable the measurement of optical intensity at specific wavelengths or spectral bands. Optical spectrometers are used in some chemical or biological analysis devices or analyzers to detect, identify and/or quantify chemical or biological species. Absorption, Raman and fluorescence spectroscopy are some of the most common methods used to optically analyze chemical or biological samples. Several commonly used optical spectrometers today include Fourier-transform spectrometers (FTS), grating based spectrometers being the most common dispersive type of spectrometer, and filter based spectrometers that employ a linear variable filter (LVF)—e.g., as described in U.S. Pat. No. 5,166,755 to Gat; U.S. Pat. No. 5,218,473 to Seddon et. al.; and U.S. Pat. No. 6,057,925 to Anthon, the texts of which are incorporated herein by reference, in their entirety—and angularly tuned filter spectrometers—e.g., as described in U.S. Pat. No. 4,040,747 to Webster; U.S. Pat. No. 2,834,246 to Foskett; U.S. Pat. No. 5,268,745 to Goody; and U.S. Pat. No. 7,099,003 to Saptari, the texts of which are incorporated herein by reference, in their entirety.
Many chemical and biological analyses today require high sensitivity devices, measuring trace components very accurately and reproducibly, down to parts-per-million or even parts-per-billion levels. As such, optical spectrometers with high optical throughput or etendue are essential. Fourier transform spectrometers and angle-tuned filter spectrometers, in principle, can provide the required high optical throughput, whereas dispersive spectrometers provide relatively much lower throughput. In addition, many chemical and biological analyses require multiple and/or wide spectral or wavelength coverage due to the need to measure multiple species and/or the need to compensate for signal interferences arising from the presence of other species in the sample.
Although Fourier transform spectrometers can in principle provide high throughput or etendue capability, they require complex instrumentation, including high-precision optical components and subassemblies, and thus are relatively expensive and cumbersome to operate and maintain. Rotating filter spectrometers, such as those described in U.S. Pat. No. 4,040,747 to Webster; U.S. Pat. No. 5,268,745 to Goody; and U.S. Pat. No. 7,099,003 to Saptari, can provide as high throughput as Fourier transform spectrometers without the instrumentation complexity of the FTS. However, the tilting or rotating filter spectrometers provide rather limited wavelength coverage, for example, approximately, 1-5% of the nominal wavelength of the filter, corresponding to an effective angular scan distance of approximately 0-60 degrees, due to geometrical limitations (zero degree refers to zero degree of incident angle).
Systems employing a plurality of filters have been described and are reported to include additional wavelength bands and/or extend the wavelength coverage. See, e.g., U.S. Pat. No. 4,040,747 to Webster; and U.S. Pat. No. 7,099,003 to Saptari. However, such systems provide additional wavelength bands by adding filters that are scanned in series, i.e. one wavelength band is scanned at a time. With such systems, the number of wavelength bands that can be covered by the system is limited geometrically by the rotating assembly design. For example, with a single rotation axis and a single rotation assembly, the maximum practical number of filters, and thus wavelength bands, is three (e.g., see U.S. Pat. No. 4,040,747 to Webster; and U.S. Pat. No. 7,099,003 to Saptari). Additional bands may be achieved through the use of additional filter assemblies and/or axes of motion. However, such additions would further complicate the system instrumentation. In addition, such filter spectrometers do not exhibit the multiplex advantage of FTS, i.e. being able to measure the analysis wavelength bands simultaneously, which provides a further improved sensitivity performance. There is a need for a spectrometer that provides the multiplex advantage of FTS, with reduced instrumentation complexity.